The Medium allows themselves to be possessed by the angry, condemned spirits which are hovering at the edge of the underworld, trying to break back into the mortal realm. These spirits have been driven mad by their ceaseless torure in The underworld, so they are eager for any opportunity to catch a glimpse of the realm of the living. So, in exchange for a trip down memory lane as it were, the tortured soul Possesses the body of the Medium and imbues him/her with their various powers.
Attributes:
Soul Capacity: Increases the duration of all Possession spells and Weapon spells by 3% per attribute level (p)
Hate Magic: No Inherant effect. Many skills, especially those used to Damage or deter foes, become more effective with each rank in this attribute.
Restraint: No inherant effect. Many skills, especially those used to reduce the penalties tied to spells or restore energy, become more effective with each point in this attribute.
Possession Magic: No inherant effect. Many skills, especially those used to take the form of a spirit, become more effective with each point in this attribute.
Skill Examples:
Horrify: Spell. If target foe was attacking they are knocked over, and any stance thay were using is ended. (Att: Hate Magic)
5 energy, 1/4 sec cast time, 12 sec recharge
"Feed Me!": shout. all nearby foes suffer from bleeding for 3-9 seconds and you gain 10-30 health for each foe affected. (Att: Hate Magic)
10 energy, instant cast, 45 sec recharge
Calming Melody: Skill. For 8 seconds you sacrifice 15-40% less health when using sacrificial spells.(Att: Restraint)
10 energy, instant cast, 10 sec recharge
Protective bindings: Enchantment. For 3-18 seconds you move 30% slower but cannot suffer from new conditions. Whilst under this enchantment All hexes take twice as long to cast on you. (Att: Restraint)
10 energy, 1 sec cast time, 30 sec recharge.
Plagueling form: Possession spell. For 10-40 seconds you take the form of a Plagueling. All your attacks cause disease for 8 seconds and enemies adjacent to you suffer 1-3 health degeneration. Whilst in this form, your off hand item is disabled.(Att: Possession Magic)
25 energy, 3 sec cast time, 60 second recharge.
Harbingers form: Elite Possession Spell. For 15-45 seconds you take the form of The Harbinger. You gain 5 base damage reduction and all your attack skills cause Knock-Downs. (Att: Possession Magic)
25 energy, 3 sec cast time, 100 second recharge.
The Medium will have a basic energy pool of 30, same as most spellcasters, and will have 65 max armour, just to break the decimal trend.
Possible armour mods could be extra armour or energy whilst in a Form.
Medium armour might be straight-jackets or bandages, perhaps stained with vomit or blood (Gross-out FTW!) and maybe those little metal discs doctors wear on their heads for headgear.
Animations could include cupping their foreheads and sinking to their knees, or balling their fists and screwing up their faces.
So please post your opinions and critisisms, i hope to do some pictures for tommorow to help show the way the Medium should look.

:Edited to avoid overpowering: